In this section we examine in detail the impact of employing steep slope technology in different domains ranging from the ultra-low power domain with energy harvesting to the server/high peformance domain with 3D stacked multicore architectures.
\subsection{RF Energy Harvesting using Steep Slope Devices}

\begin{figure}[ht!]
\epsfig{file=figs/energy_harvesting.eps, angle=0, width=1.05\linewidth, clip=}
\caption{\label{fig:energy-harvesting} Embedded systems powered by energy harvesting sources}
\end{figure}

%The energy density advance of batteries has lagged behind the power scaling of integrated circuits and systems. However, batteries are are still prevalent power sources nowadays, and the necessity of replacing and charging of these batteries limits the design space of wireless circuits and systems. At the same time, energy sources like solar cells [1]-[3], thermo-electrics [5], piezo-electrics [8], and RF signals [6][7] have been studied further to power low-power systems. In recent years, emerging battery-less designs, such as RFID [9][10], RF-powered insect monitoring and sensing [11][12], vibration-powered transceiver  [13], have extended the traditional design space based on wiring and batteries. 
Improvements in the energy density of batteries have lagged behind the power scaling of integrated circuits and systems. However, batteries are are still prevalent power sources nowadays, and the necessity of replacing and charging of these batteries limits the design space of wireless circuits and systems. 
At the same time, energy sources like solar cells, thermo-electrics, piezo-electrics, and RF signals have been studied further to power low-power systems. In recent years, emerging battery-less designs, such as RFID, RF-powered insect monitoring and sensing, and vibration-powered trans-ceivers have extended the traditional design space. 

Table~\ref{tab:energy-sources} summarizes the characteristics of typical non-battery power sources along with the traditional lithium ion battery. 
It can be concluded from Table~\ref{tab:energy-sources} that, under certain conditions, the non-battery sources are able to generate power up to several mW. It is also important to notice that the applications using such non-battery power sources are limited in that they have strong dependence on the environment, e.g. the light intensity, vibration strength, temperature gradient, and RF power density. 
Hence, due to limited and unstable power that could be harvested from the ambient environment, existing battery-less systems have a limited operation range or computational capability. 
For example, with a wireless several-watt RF signal transmitter, existing RFID tags typically have an operation range of meters and support only primitive sensing,  identification, or transmission~\cite{UCSDRectifier}.%[11][12][13]. 
Therefore, aside from the exploration of more power-efficient energy harvesters, the synergy of hybrid power sources and the more power-efficient signal processing modules based on TFETs is of great significance. 

Fig~\ref{fig:energy-harvesting} shows an embedded system powered by hybrid energy sources. Using a smart power management module to obtain co-operation of two or more power sources, the system operation is less likely to be interrupted. 
Furthermore, it has been already shown that steep-slope TFET has higher power efficiency in both digital and analog signal processing domains. 
Finally, energy harvesters like the AC-DC rectifiers based on TFET also have high power-conversion-efficiencies~\cite{HuichuISLPED2013Rectifier}. The design space of TFET-based system is thus significantly extended through the high power-conversion-efficiency energy harvester, high efficiency signal processing, and the synergy of hybrid power sources. 

\begin{table}[ht!]
\small
\begin{minipage}[c]{1\linewidth}
\begin{center}
%\vspace{-0.2cm}
\begin{tabular}{|c|c|c|} \hline
\textbf{Energy} & \textbf{Output power}  & \textbf{Cost, Volume}	\\
\textbf{Source} & \textbf{and voltage} & \textbf{Area etc.} \\\hline
\multirow{4}{*}{Solar cells} & Efficiency 8-28\% & \$0.6/Watt\\
			     & $\sim$700mv for 0.1W/$cm^2$ & Silicon thickness: \\
			     & power density by & 30-100~$\mu$m	\\
			     & crystal-Si solar cell & \\\hline
Lithium  & 1.8 W/g, 4V & 0.3-0.8/Wh \\
Ion Battery &  & size down to a coin \\\hline
Thermo- & ~20$\mu W/cm^2$  & 1300 thermocouples  \\
Electrics &  extracted from body & per $cm^2$ \\\hline
\multirow{5}{*}{RF} 	& \multicolumn{2}{l|}{Typical values:} \\
		 	& \multicolumn{2}{l|}{a) 25(100)$\mu W$ $\sim$20(10)km from 1MW TV station} \\
   			& \multicolumn{2}{l|}{b) 5~$\mu W$ $\sim$100 m from 40W cellular station} \\
			& \multicolumn{2}{l|}{c) 45(4500)$\mu W$ $\sim$10(1.0)m } \\
			& \multicolumn{2}{l|}{from 4W RFID reader with $R_x$ antenna.} \\\hline
Piezo-electrics &  2.1~mW/$cm^2$ at 4 km/hr\\\hline
\end{tabular}
\caption {Typical energy sources for embedded systems}
\label{tab:energy-sources}
%\vspace{-0.2in}
\end{center}
\end{minipage}
\end{table}
